Neuropeptide Y is a peptide present in the central and peripheral nervous systems. The peptide co-exists with noradrenaline in many neurons and acts as a neurotransmitter per se or synergistically together with noradrenaline. Neuropeptide Y-containing fibers are numerous around arteries in the heart, but are also found around the arteries in the respiratory tract, the gastrointestinal tract, and the genitourinary tract. Neuropeptide Y is also present in the cerebrum with effects on blood pressure, feeding, and the release of different hormones. Alterations in central concentrations of neuropeptide Y have been implicated in the etiology of psychiatric disorders.
Neuropeptide Y was discovered, isolated and sequenced about ten years ago from porcine brain as part of a general screening protocol to discover carboxy-terminal amidated peptides and was named neuropeptide Y due to its isolation form neural tissue and the presence of tyrosine as both the amino and carboxy terminal amino acid. Neuropeptide Y is a member of the pancreatic family of peptides and shares significant sequence homology with pancreatic polypeptide, and peptide YY.
Neuropeptide Y and the other members of its family of peptides all feature a tertiary structure consisting of an N-terminal polyproline helix and an amphiphilic .alpha.-helix, connected with a .beta.-turn, creating a hairpin-like loop, which is sometimes referred to as the pancreatic polypeptide (PP) fold. The helices are kept together by hydrophobic interactions. The amidated C-terminal end projects away from the hairpin loop.
Subsequent to its discovery neuropeptide Y was identified as being the most abundant peptide in the central nervous system with widespread distribution including the cortex, brainstem, hippocampus, hypotahlamus, amygdala, and thalamus as well as being present in the peripheral nervous system in sympathetic neurons and adrenal chromaffin cells.
Neuropeptide Y seems to fulfill the main neurotransmitter criteria, since it is stored in synaptic granules, is released upon electrical nerve stimulation, and acts at specific receptors. It is clear that neuropeptide Y is an important messenger in its own right, probably in the brain, where neuropeptide Y potently inhibits the activity of adenylate cyclase and induces an increase in the intracellular levels of calcium. Central injection of neuropeptide Y results in blood pressure changes, increased feeding, increased fat storage, elevated blood sugar and insulin, decreased locomotor activity, reduced body temperature, and catalepsy.
Neuropeptide Y (as well as its chemical relatives) acts upon membrane receptors that are dependent on guanine nucleotides, known as G protein-coupled receptors. G proteins are a family of membrane proteins that become activated only after binding guanosine triphosphate. Activated G proteins in turn activate an amplifier enzyme on the inner face of a membrane; the enzyme then converts precursor molecules into second messengers.
Neuropeptide Y appears to interact with a family of closely related receptors. These receptors are generally classified into several subtypes based upon the ability of different tissues and receptors to bind different fragments of neuropeptide Y and the closely related peptide YY. The Y1 receptor subtype appears to be the major vascular neuropeptide Y receptor. The Y2 receptor subtypes can also occur postjunctionally on vascular smooth muscle. The as-yet-unisolated Y3 receptor subtype appears to be neuropeptide Y-specific, not binding peptide YY. This receptor is likely to be present in the adrenal tissues, medulla, heart, and brain stem, among other areas. For a review of neuropeptide Y and neuropeptide Y receptors, see, e.g., C. Wahlestedt and D. Reis, Annual Review of Pharmacology and Toxicology, 33:309-352 (1993)!.
In view of the wide number of clinical maladies associated with an excess of neuropeptide Y, the development of neuropeptide Y receptor antagonists will serve to control these clinical conditions. The earliest such receptor antagonists were peptide derivatives. These antagonists proved to be of limited pharmaceutical utility because of their metabolic instability.
In essence, this invention provides a class of potent non-peptide neuropeptide Y receptor antagonists. By virtue of their non-peptide nature, the compounds of the present invention do not suffer from the shortcomings, in terms of metabolic instability, of known peptide-based neuropeptide Y receptor antagonists.